1. Field of the Invention
The present invention relates to an in-line screw type injection molding machine, and more particularly to a check valve mechanism provided at the tip end of a screw, for preventing resin from flowing backward.
2. Description of the Related Art
An injection molding machine obtained by providing one screw with functions of plasticizing (melting) material, kneading and injection is referred to as an in-line screw type injection molding machine. In ordinary cases, the in-line screw type injection molding machine has a check valve mechanism, at the tip end of the screw, for preventing resin during injection from flowing backward. FIG. 1 shows an example of this check valve mechanism. In a portion reduced in the diameter between a screw head 2 provided at the tip end of a screw 1 and the body portion of the screw 1, there is arranged a check ring 3 so as to be moveable in a direction of a screw axis, and further on the body side of the screw 1 of this reduced-in-diameter portion, there is arranged a check seat 4 for closing a resin path by abutting upon and being brought into tight contact with this check ring 3.
Resin pellets to be supplied from behind the screw 1 are melted by means of shearing heat to be generated by the rotation of the screw 1 and heat from a heater provided in the outside of a barrel in which the screw 1 is inserted. The resin thus melted raises resin pressure behind the check ring 3 to cause a force for pressing the check ring 3 toward the front. When the check ring 3 is pressed toward the front, the resin in the rear passes through a clearance between the check ring 3 and the reduced-in-diameter portion, and flows in front of the check ring 3 to raise pressure within the barrel in front of the screw head 2.
When pressure in front of the check ring 3 exceeds predetermined pressure (back pressure), the screw 1 is pressed toward the rear and the pressure in front of the check ring 3 is reduced. Further, the screw 1 is rotated, whereby pressure behind the check ring 3 becomes higher than the pressure in front of the check ring 3, and therefore, continuously-molten resin is fed toward the front of the check ring 3. When the screw 1 retreats up to a predetermined amount, the rotation of the screw is stopped.
Next, an injection process starts. When the screw 1 advances in order to fill up resin, resin pressure accumulated in front of the screw head 2 is raised, and therefore, the check ring 3 retreats, and is brought into tight contact with the check seat 4 to close the resin path. Filled-up pressure prevents the molten resin from flowing backward in a screw retreat direction. When there fluctuates timing whereat the check ring 3 retreats to close the resin path, an amount of resin to be filled also fluctuates so that molding becomes unstable.
The check valve mechanism during injection is closed when the pressure in front of the check valve mechanism becomes higher than the pressure in the rear because of an advance of the screw 1, and in the rear of the check valve mechanism immediately before injection as described above, there is residual pressure in a grooved portion 6 between flights 5. There is a problem that closing timing fluctuates under the influence of this residual pressure. Thus, there have been proposed various methods such as means capable of closing the check valve mechanism for each cycle with stability and a method for determining timing whereat the check valve mechanism actually closes, and controlling an injection process on the basis of the timing thus obtained.
For example, there have been known a method (See Japanese Patent Application Laid-Open No. 4-201225) by which a pressure sensor is added within a cylinder, closing of the check valve is detected on the basis of a change in pressure during the advance of the screw, and a point in time of closing thereof is used as a starting point of an injection stroke, a method (See Japanese Patent Application Laid-Open No. 3-92321) of detecting a position of a ring valve by taking advantage of electrostatic capacity, a method (See Japanese Patent Application Laid-Open No. 51-73563) by which the closing of the check valve is detected by detecting leading edge of injection pressure at the commencement of injection to control an injection stroke from the detection position, and the like.
Also, there have also been known a method (See Japanese Patent Application Laid-Open No. 62-60621) of closing the check valve by reversing the screw forcibly or by releasing the fixation to close the check valve concurrently with the commencement of injection, and a method (See Japanese Patent Application Laid-Open No. 11-170319) by which in order to prevent the screw from reversing at the commencement of injection, the brake is applied to start injection, and after the check valve is closed, the brake is released, whereby wear is reduced to extend the screw life.
Further, there have been proposed various methods by which after the completion of metering before injection is started, resin residual pressure in a groove portion of the flight portion is reduced, whereby the check valve is caused to close concurrently with the commencement of injection. For example, there have been known a method (See Japanese Patent Application Laid-Open No. 2000-858) of reducing pressure in the flight portion by reversing the screw after the termination of metering, and a method (See Japanese Patent Application Laid-Open No. 60-76321) of closing the check valve by performing preliminary injection to control the injection process on the basis of a screw position after this preliminary injection. Also, there is also known a method (See Japanese Patent Application Laid-Open No. 6-71706) by which an injection screw is brought into a freely rotatable state for injection, and closing of the check valve is facilitated by rotation of the injection screw.
In order to detect, by an increase in resin pressure, that the check valve mechanism is closed, it is necessary to add a pressure sensor, and there is a problem that the mechanism will become complicated. Also, in the method of reducing the pressure before the commencement of injection after completion of metering, there is a defect that control for this pressure reduction process becomes complicated.
Also, in the method of reversing the screw, described in the Japanese Patent Application Laid-Open No. 6-71706, since an amount of inverse rotation is adjusted by a mechanical part of an engaging protrusion of a spline ring to be installed, there is a problem that the structure becomes complicated. Also, in the method of performing the inverse rotation by releasing the fixation of screw rotation concurrently with the commencement of injection, described in the Japanese Patent Application Laid-Open No. 62-60621, since the screw rotation remains released at all times during injection/dwell, there is a problem that when pressure at the tip end of the screw has been reduced while the dwell pressure is controlled, the screw rotates in a normal direction, resin is supplied toward the front, and the amount of fill-up fluctuates.